Synthetic plastic polymers are normally sold in a granular or pellet form for further processing into articles of various shapes and forms for ultimate use by the consuming public. Such articles may be films, fibers, bottles, containers, strips, rods, and molded articles. The processing which transforms the pellets into the final article usually involves heating the pellets to form a molten, viscous liquid mass which is then passed through devices to solidify the molten mass into a final shape. The normal type of process machinery to accomplish this transformation is a screw extruder which introduces friction, heat and pressure to melt the pellets and causes good mixing in the molten mass as it is pushed through the extruder. While in the molten phase there may be any of several additives incorporated therein, e.g. dyes or pigments for coloring, ultraviolet light absorbers for protection against deterioration by sunlight, antioxidants to decrease or eliminate deterioration by oxygen in the air, and lubricants to facilitate the processing of a highly viscous melt which sticks tenaciously to surfaces of the processing equipment. In some instances the additive is mixed with the pellets at the hopper, but in other instances it is not feasible to do so.
In the case of a waxy additive it cannot be introduced at the hopper because of its adverse effect on the normal mixing and frictional requirements in a screw extruder. The screw turns in a stationary barrel causing the pellets to be compressed against the barrel and to produce large amounts of frictional heat which enhances the melting of the pellets. This effect is basic to an extruder. When the waxy additive is introduced through the hopper it materially reduces the frictional drag at the barrel and delays or materially prohibits the proper melting of the pellets and the throughput of the extruder. On the other hand, the surface of the screw also produces frictional heating, causing the pellets to move along the screw as they change into the molten phase and exit from the extruder. It is particularly desirable for the melting to occur at the barrel surface and not at the screw surface. Accordingly, there has been a need to provide a method for adding a waxy material, e.g., a lubricant at the interface of the pellets and the screw, but not at the interface of the pellets and the barrel. The present invention meets this need in the plastic extruder art.
Within the scope of this invention a screw extruder encompasses not only the commonly known extrusion devices, but also any apparatus used to feed or blend additives introduced into plastic materials, and other forming apparatus for plastic materials using the principle of a rotating screw. Some injection molding apparatus and screw rams employ such screw devices and are to be included within the term "screw extruder" in this invention.
It is an object of this invention to provide an improved process for preparing an additive modified polymeric extrudate. It is another object of this invention to provide a method for incorporating a waxy material such as a lubricant, into a plastic extrudate with a substantial reduction in the power consumption of the extruder. A further object is the provision of an additive to a plastic extruder which lubricates the screw and not the barrel until substantially all the plastic pellets are melted or until mixing of the plastic pellets occurs downstream of the feed section whereby the throughput of the machine is enhanced in the feed section as well as in the melt section thereof. Still other objects will become apparent from the more detailed description which follows.